Climate change sealed the fate of ichthyosaurs, marine reptiles that ruled the oceans for 157 million years, suggests an analysis of fossils.

The dolphin-like animals died out some 30 million years before the mass dinosaur extinction at the end of the Cretaceous 66 million years ago.

Vertebrate palaeontologist Dr Valentin Fischer, who led the research, published today in Nature Communications, said that the extinction of ichthyosaurs, which were extremely well adapted to oceanic life, was a long-standing enigma.

A number of hypotheses have previously been proposed to explain the ancient marine reptiles disappearance, including increased competition from other marine reptiles and fish and the decline of their main food source, squid-like belemnites, said Dr Fischer of the University of Oxford.

Over time, these pressures would have reduced species diversity, allowing relatively minor events to tip them into extinction.

"These theories were at odds with the recent understanding of the ichthyosaur fossil record, which suggests they were actually quite diverse prior to their extinction," he said.

"We wanted to analyse this extinction thoroughly and look for possible alternative drivers."

Instead, their demise was associated with strong fluctuations in sea levels and temperatures during what is known as the Cenomanian stage of the upper Cretaceous period, 100 million to 94 million years ago.

"It seems that a large part of the marine biosphere was affected by an event or a series of rapid events that profoundly modified marine ecosystems," he added.

"While this taxonomic proliferation does infer periods of higher diversity, it is restricted by the relatively small area of globally explored outcrops, especially in the southern hemisphere," said Dr Kear, who is currently the curator of vertebrate palaeontology at Uppsala University's Museum of Evolution.

"This means that our sampling is very incomplete, both in terms of geography and time - which limits clarity of the results.

"Having said that, this research opens an exciting new line of inquiry that will fuel further investigations into the ichthyosaur extinctions and the emergence of modern marine predators such as sharks and bony fish during the Cretaceous," said Dr Kear, who was not involved in the study.

We all enjoy a tall tale. Cultures with seafaring traditions are especially ripe in what seem like the tallest sea monster tales of all: hydra, kraken, sirens, scylla, leviathans, assorted serpents and mermaids.
Usually the stories are never confirmed and deemed baseless. Then again, some of the tales are based on something. With our skeptical hats on, let's have a look at sea monsters both real and fanciful.
We begin with a story that went viral just this week, about a supposed monster that revealed itself during a swim in the Thames River.
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really can, and will, use their heads for ramming -- just as the fictional Moby Dick did, in the Herman Melville classic of the same name. Did whales perfect the head-butt long before people started banging heads?

Sea monsters are truly global, of course. This one from Japan serves as the villain for the classic maiden in distress, who awaits rescue by her hero. The poor monsters are almost always cast as the bad guys. And so they usually end hacked to pieces; fish food.
But is there any truth behind these sea serpent tales?

Maybe it's the oarfish. It looks too monstrous to be true. It can grow many meters long, has strikingly bright silver scales, scarlet fins and some ornate headgear that more than explains why some call it a roosterfish. If only it were a reptile, it'd be a true sea serpent.
Alas. It is a fish. A very weird and beautiful fish, but still a fish.

There are also other, newfound "sea serpents" our sea-going ancestors never imagined. This one was spotted by a satellite coiling off the south coast of Japan's Hokkaido island.
What do we know about it? 1) It's arguably one of the largest organisms on Earth, 2) It swallows ships, engulfs islands and generally does what it wants, and 3) We're darned lucky it's made of plankton.

Research into such massive blooms and the individual plankton cells that comprise them has revealed surprising cooperation among the microorganisms. They appear to operate like more than just floating individual cells. They live and die for the greater good, it seems. So they may be, in fact, a gigantic watery superorganism.
Now that's a cool monster for you: You can swim in it and never know you've been in the belly of a beast.

Mermaids and mermen have always been the stuff of fantasy. Where did the fantasies come from? There are some standard answers to this question, which have always seemed rather inadequate. For instance ... (next slide, if you please) ...

The manatee has often been called the source of mermaid myths. It's a mammal, so it breathes air. But who would ever mistake a manatee for a sleek and beautiful mermaid?
Could it be love-starved sailors with poor eyesight? There was no shortage of these fellows in the days before optometrists.

How about giant, ship-destroying squid and octopi? These monsters were old hat even to the easily freaked-out. Most folks figured they were historical exaggerations.
That's until some very large and unusual squids started washing up or being hauled in by marine biologists in recent years. Colossal squid are meters long, pretty amazing beasts. Still, they have never been known to lift ships out of the water.
And since were on the topic of squids ...

Do you remember when this one hit the headlines? It's not so gigantic, at four meters long, but it was observed 3,380 meters down in the Pacific Ocean near Oahu. It's pretty big to have gone unseen before its May 2001 discovery.
So what else is out there? It's pretty clear marine biologists have only just begun discovering what lives in the deep sea. The more time they spend searching, the more they will find. But none would dispute that the nastiest sea monster to ever rise out of the sea is ... (drum roll please) ...

You might have guessed it: Human garbage. Yep. It's the ugliest, most alien-looking, fatal and pervasive monster in the seas. Garbage patches have been getting a lot of attention lately. These are areas on the seas where currents and winds tend to concentrate floating garbage.